From flushing flows to (eco)morphogenic releases: evolving terminology, practice, and integration into river management

Abstract

Abstract Flushing flows are deliberate high-flow releases designed to mimic effects of floods in removing fine sediment from downstream aquatic habitats. A special case of environmental flows, flushing flows are intended to act on geomorphic processes to mitigate ecological effects of dams. We review definitions advanced for these flows, and then propose a new term, morphogenic releases, for high flows designed to interact with the channel to produce biophysical changes in channel characteristics. Similarly, ecomorphogenic releases is proposed for those flows intended specifically to improve aquatic and riparian habitats. We then review objectives of such specific environmental flows and potential conflicts with environmental, socio-economic, and dam-operational issues. We also provide some recommendations based on our literature review and experiences to specify the dimensions of morphogenic flows. Experiments to assess effects of morphogenic flows are increasingly part of the process of defining environmental flows. We highlight the challenges to implement morphogenic flows and some alternative actions when such flows cannot be implemented, recognizing that flow is not the only variable of interest downstream of dams to mitigate effects of dams; that mitigation of channel form, sediment supply, water quality, or other factors may also be important. Returning the channel to a pre-regulation state is often not possible, and it is necessary to address issues pragmatically by identifying the most efficient improvement options and what is financially feasible and socially desirable.